Hydrogen bonding and long-range proton coupling constants in some ortho disubstituted aniline derivatives. Intramolecular proton exchange and degree of nonplanarity of the amino group

Abstract

Long-range spin–spin coupling constants over five bonds between the two amino protons and the ring protons are reported for some ortho disubstituted aniline derivatives in cyclohexane and benzene-d6 solutions. Equal coupling of both amino protons to the meta protons in unsymmetrically ortho disubstituted aniline derivatives indicates the occurrence of intramolecular exchange of the amino protons. The absence of any observable coupling over six bonds to the para proton places an upper limit on the degree of nonplanarity of the amino group in the 2,6-dibromo- and 2,6-dichloroanilines. A relatively basic compound like 2,4,6-trimethylaniline does not display long-range couplings of the amino protons, very likely because intermolecular proton exchange occurs.

Long-range Spin–Spin Coupling Constants from Amino Protons and 15N to Ring Protons in Aniline-15N and Some Derivatives. INDO Molecular Orbital Calculations

“…Because of vroblems with intermolecular proton exchange, this parameter was not observable at that time (22) but the prediction is now borne out by the observed value of -0.137 Hz ( Table 1 ). The prediction assumes the geometry of the aniline molecule in the gas phase (33), having an angle 4 of 20" between the ring plane and the plane defined by the nuclei in the C-N-H fragment (see 6~h e r e is a much larger difference between calculated and observed values when the acetyl group is bonded directly to the nitrogen atom, as in acetanilide (Tables I and 4).…”

“…Such couplings for the amino protons therefore offer another approach to the study of the geometry of this moiety. The couplings are less than 1 Hz in magnitude and this approach has until recently (22)(23)(24) been hampered by the facile intermolecular exchange of 'NRCC Scholarship, 1971-72. ZPostdoctoral Fellow, 1971 the amino protons in solutions of aniline and its derivatives.…”